Investigation of the effects of lipoic acid and salicylic acid applications on some biochemical parameters and proteome changes in canola seedlings under salinity stress

In this study, the protective potential of exogenously applied lipoic acid (LA) and salicylic acid (SA) in inducing salinity tolerance in hydroponically grown canola (Brassica napus L. cv. Sary) seedlings. For this purpose, 18 day-old canola seedlings divided into six treatment groups (control, LA, SA, NaCl, NaCl+LA and NaCl+SA), and they were grown another seven days. The leaves of seedlings were sprayed every alternate day with 0.1 mM of LA or SA. Following these treatments, the seedling growth, chlorophyll content, malondialdehyde (MDA) content, proline content, cysteine content, non-protein thiol (NPT) content, antioxidant [ascorbate (AsA) and glutathione (GSH)] contents and activities of some antioxidant enzymes [superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX)] were determined in 25-day-old canola seedlings. Salinity stress decreased the growth of shoots and roots and contents of AsA and GSH, and increased the contents of malondialdehyde (MDA), proline, cysteine and NPT and the activities of antioxidant enzymes such as SOD, POD, CAT and APX. However, exogenous applications of LA or SA alleviated the salt stress-mediated seedling growth and decreased the MDA content. LA or SA applications could not influence the proline content. While the LA and SA applications more increased the cysteine content, decreased NPT content in seedlings under salinity stress. Exogenous application of LA further reduced AsA content in salt-stressed seedlings, while SA had no significant impact in this respect. Under salinity stress, only SA application improved the GSH content. Exogenous applications of LA or SA increased the activities of POD and CAT, while these applications caused reduction in the APX activity of the salt-stressed seedlings. These results suggest that under short-term salinity stress, LA is at least as effective as SA for improving growth by influencing defense systems. In the second stage of this study, the influence of exogenously applied LA on the salt tolerance mechanism in seedlings of canola was investigated using proteomic approach. For this purpose, proteins from the leaf tissues of canola seedlings subjected to control, LA, NaCl and NaCl+LA treatments were identified by two-dimensional (2-D) gel electrophoresis [isoelectric focusing (IEF)/Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS/PAGE)] and MALDI-TOF/TOF mass spectrometer. Among the 28 proteins exhibiting alterations in expression pattern, 22 proteins were identified. Among them, 17 proteins were up-regulated and 5 proteins were down-regulated. The expression levels of 5 proteins (photosystem II reaction center PSB28 protein, thioredoxin M-type 4, 50S ribosomal protein L12-3, 2-Cys peroxiredoxin BAS1 and sedoheptulose-1,7-bisphosphatase) were regulated by only LA treatment. However, the expression levels of carbonic anhydrase, magnesium-chelatase subunit chli, RuBisCO activase, triosephosphate isomerase and thiazole biosynthetic enzyme ware down-regulated by NaCl stress or LA application. The identified proteins were involved in various cellular responses and metabolic processes such as photosynthesis, energy metabolism, stress defense and signal transduction. The revealing of the protective roles of LA in canola seedlings under salt stress by proteomics study provides new insight to evaluating protein functions in salt tolerance mechanisms.